The invention relates to a method for polarization mode dispersion compensation, and to a polarization mode dispersion compensator.
Optical waveguide transmission paths which are as long as possible are used in optical transmission technology. The optical waveguides are not completely isotropic owing to the way in which they are produced. As a result of the long transmission paths, birefringence of the optical signals which are transmitted results in frequency-dependent polarization transformationxe2x80x94referred to as polarization mode dispersion PMD, or polarization dispersion for short. As a result of the change in polarization and the different frequency-dependent propagation times resulting from this change, the change in propagation leads to transmitted pulses being broadened, which makes it harder to identify them at the receiving end, and thus limits the data rate which can be transmitted.
The polarization mode -dispersion is also dependent on the temperature or on mechanical stress. Adaptive PMD compensators are therefore required, which are inserted in the transmission path. The aim of a PMD compensator is to make the polarization transmission response of the overall system comprising the transmission path and the compensator approximately (to a first or, if required, even higher order) frequency-independent for at least one optical carrier frequency in the region of the transmission band. Modulated signals can thus be transmitted without distortion.
For wavelength division multiplexing WDM, it is desirable to achieve this frequency independence for the individual transmission bands (transmission wavelengths) at least in each of the individual channels. The requirements for such a transformer/compensator are low insertion loss, compatibility with optical waveguides, that is to say low coupling loss and mechanical compatibility, and a polarization behavior which is frequency-dependent and is as variable as possible.
PMD emulators whose adjustment can be varied but which are nevertheless low in price and low in attenuation, and which can simulate in a significant manner the frequency-dependent polarization transmission behavior of optical waveguide distances of lengths of up to several thousand kilometers and in widely differing conditions (for example with temperature fluctuations) are required in order to develop PMD compensators and in order to check the PMD tolerance of uncompensated transmission systems.
A xe2x80x9cTransmission System and Receiver with Polarization Controlxe2x80x9d is known from Patent Application WO 95/34141, which uses an LiNBO3 crystal whose XY plane runs transversely to the propagation direction Z. The propagation speeds in the X and Y axes are initially constant. However, they become slightly different by application of control voltages, so that the polarization can be varied. However, as a rule, polarization transformers are unsuitable for PMD compensation since, in this case, it is necessary to correct for major propagation time differences between the individual modes. These may amount to a duration of from approximately ⅓ of a bit up to several bits.
Compensators whose transmission characteristics are therefore the inverse of a transmission path are suitable to compensate for PMD. Compensators are known from the literature, which are in the form of retarders/polarization rotators and are arranged between relatively strongly birefringent pieces of optical waveguide. Retarder is a generic term for optical transmission elements which transmit two mutually orthogonal eigen modes but with phase delays which are in general different.
The strongly birefringent optical waveguide sections maintain or ensure two mutually orthogonal main polarizations and are thus polarization-maintaining optical waveguides PMF (polarization maintaining fibers). These PMFs are strongly polarization-dispersive, that is to say different polarizations lead to widely differing propagation times. An appropriate example is described in the proceedings of the xe2x80x9cOptical Fiber Communication Conferencexe2x80x9d, 1995, OFC""95, the Optical Society of America, as article WQ2 on pages 190 to 192.
An integrated optical single-sideband modulator and phase shifter is described in xe2x80x9cIEEE Journal of Quantum Electronicsxe2x80x9d, Volume 18, No. 4, April 1982, pages 767 to 771. This device contains, on a lithium-niobate substrate, a ground electrode which is in the form of a comb and is drawn over the chip, and electrodes which are in the form of combs, lie in a row and whose tines are interleaved with the tines of the ground electrode, and every alternate one of which is respectively connected to a first control voltage or a second control voltage, respectively. In this polarization transformer, the TE-TM mode conversion can be preset only with xc2x145xc2x0 linear-polarized eigen modes, or with circular-polarized eigen modes. In this case, the TE-TM phase shift element is governed by the chip length and the chip temperature and cannot be varied by an electrical voltage. Another disadvantage of this arrangement is that a preset polarization transformation is effective for only one specific optical frequency, that is to say the frequency dependence of polarization transformers cannot be preset freely.
An integrated optical polarization transformer which uses lithium niobate LiNbO3 or lithium tantalate LiTAO3 as the substrate is described in xe2x80x9cIEEE Journal of Quantum Electronicsxe2x80x9d, Volume 25, No. 8, Aug. 8, 1989, pages 1898 to 1906. This requires only three different control voltages, one phase-shifter voltage and two mode-converter voltages in order to produce any desired polarization change. The phase-shift voltage produces a phase delay between TE (transverse electrical) and TM (transverse magnetic) waves, which are at the same time the eigen modes, but does not produce any conversion between them. One of the two mode-converter voltages produces TE-TM mode conversion with linear polarization at xc2x145xc2x0 elevation angles as eigen modes, and the other produces TE-TM mode conversion with circular polarization as eigen modes. However, a predetermined polarization transformation is effective for only specific optical frequency. At other optical frequencies, the polarization transformation is dependent on the polarization transformation which is set for this specific optical frequency.
A TE-TM transformer with simple electrode shapes is described in the Proceedings of the Fourth European Conference on Integrated Optics ECIO 87, Glasgow, Scotland, Editors Wilkinson and Lamb, pages 115 to 118.
These known arrangements are used for polarization transformation, for example as polarization compensators and receivers. They are not planned for use as PMD compensators.
A method for PMD compensation is likewise described in Electronics Letters, Feb. 17, 1994, Volume 30, No. 4, pages 348 and 349. In this case, a number of sections of polarization-maintaining fibers (PMF) are used as transmission elements and are connected by means of polarization transformers, with a polarization transformer with a downstream polarization-maintaining fiber being used for PMD compensation. The attenuation which occurs may be very high owing to the spliced joints which are required.
The connection of a PMD compensator to an optical receiver and the process of obtaining a control criterion for setting the compensator are also described here. A functionally similar device has also been described on pages 258-259 of the Proceedings of the OEC""94 (Makuhari Exhibition, Japan), Article 14E-12, which achieves time differences between the modes of 28 ps.
In practice, the compensators described in the cited references are limited to very short sections of PMF. In consequence, when optimizing the control criterion, it is possible for a secondary optimum to be arrived at, so that the compensator is not set optimally.
A PMD compensator which comprises only PMFs is described in German patent application file reference 19816178.6. However, this requires mechanically moving parts.
The object of the present invention is thus to specify a method for PMD compensation and a PMD compensator which has numerous setting options and, within the transmission band in use, as far as possible allows desired frequency-dependent polarization transformation for the purpose of specific compensation.
The advantage of the PMD compensator according to the invention is that it can be used universally. The phenomenon of polarization mode dispersion, which is governed by a number of parameters, can in principle also be compensated for only by suitable adjustment options corresponding to a large number of degrees of freedom. The individual control voltages allow so many frequency-dependent polarization transformations to be set and carried out during operation that any required PMD for even those which are higher than first order, can be formed with very high accuracy, both for the purpose of PMD compensation and for PMD emulation.
Further advantages are the small physical size which is normal with integrated optical components, and the fact that the functions of polarization transformation and the production of different propagation times for different polarizations can be integrated on one component, a chip, in the PMD compensator according to the invention.
The PMD compensator can likewise be used as a PMD emulator or else as a polarization transformer.
Variants of the PMD compensator allow even more compact methods of construction.
The invention will be explained in more detail with reference to exemplary embodiments.